In a portent of an even larger Soviet military investment in Afghanistan, U.S. officials reported yesterday that the Russians are mobilizing at least two more motorized infantry divisons and gathering a large force of fighters and bombers in the area near the Soviet-Afghan border.

The continuing indications of large-scale troop movements within the Soviet Union coincided with reports from several capitals that Russian troops in Afghanistan have clashed with Afghan army elements in the capital of Kabul and several places in the countryside.

Official sources here considered it unlikely that independent-minded elements of the Afghan army would pose much of a threat to the Soviet force, now estimated at 30,000 to 40,000 combat troops.

The Afghan military, estimated by the United States at 80,000 to 90,000 men a year ago, was reported to have dwindled to no more than 10,000 to 15,000 effective troops even before the coup last Thursday which brought to power a Soviet-sponsored leader, Babrak Karmal.

Most of the remaining Afghan military has been out of sight since the coup, and U.S. anaylsts expressed doubt that more than a few garrisons with a few hundred soldiers each are still capable of functioning.

State Department spokesman Hodding Carter said that except for occasional sniping, calm appears to have returned to Kabul. Other U.S. officials said Russian troops were manning checkpoints and in control of streets in the capital, assisted by some members of the pro-Soviet wing of the Afghan Communist Party, including university students.

The civilians, wearing white armbands, were said to be handling their weapons gingerly -- and on some occasions asking passing foreigners how to operate them.

Despite the reports of sporadic fighting and sniping, no major battles were reported, according to U.S. officials.

Having taken over the capital, Soviet troops were reported to be moving by road and air to other cities throughout the country.

U.S. officials said the Russians are using at least seven airfields within Afghanistan to bring in troops and supplies. However, the intensive airlift to Kabul, which brought in the equivalent of a Soviet airborne division in more than 300 flights over a three-day period, reportedly has stopped. Some 12 to 24 Soviet flights daily are now being reported.

The troop movements within the Soviet Union may turn out to be the most significant, according to sources here. At least one motorized infantry division in Soviet Central Asia and another in Soviet Turkistan reportedly are being mobilized, possibly for movement to Afghanistan. In addition, officials reported indications that some airborne units in the western part of the Soviet Union may soon move south.

The augmentation of Soviet air force elements near the Afghanistan border is reported to include fighters and bombers in large numbers. A smaller number of Mig23 and Mig21-fighters have been reported in Afghanistan in recent days, along with Mi24 helicopter gunships and Soviet transport planes.

In New Delhi, Washington Post correspondent Stuart Auerbach quoted diplomatic sources as saying that about 25 Soviet soldiers were believed to have been killed in the overthrow of the Afghan regime last Thursday night, with 225 wounded. Washington officials said that Soviet casualties were likely, but that no reliable reports on their extent hav been received.

According to Auerbach, Afghan rebel sources in New Delhi said the Soviet presence in their country will serve to unite the contending groups of anit-government Moslem tribesmen. At least five groups are reported to be operating at present.

The rebel sources citd as evidence a demonstration yesterday at the Soviet Embassy in New Delhi that drew about 300 students protesting the Soviet troop movement into Afghanistan. Among the protester, according to the sources, were representatives of competing groups unable to unite even for demonstration in the past.

Washington sources said there was as yet no credible report of substantial military action between Soviet forces and the Moslem insurgents who have been fighting for months against the central government, especially since the Marxist takeover in Kabul in April 1978.

In the view of anaylsis here, the insurgents are likely to avoid major military engagements with the Russian troops, just as they avoided large-scale battles with Afghan regular troops in the past. But ambushes, raids and sniping are expected to continue and perhaps even to intensify because foreigners rather than Afghan forces now will be the targets.

The 30,000 to 40,000 Soviet combat troops now reported in Afhanistan, while a powerul and impressive force, is considered far from sufficient to wage a nationwide battle against insurgents who have popular backing and are spread through much of the mountainous countryside.

U.S. officials are closely watching the large-scale forces being mobilized inside Russia. If these units come down across the border and redouble the size of the Soviet expeditionary force in Afghanistan, it will be taken as a sign that the Russians have decided to all all out to crush the Afghan insurgency, regardless of the potential price.